This invention relates to an automotive engine exhaust system designed to reduce hydrocarbon emissions caused by the evaporation of fuel from the fuel tank and by the inefficiency of combustion and catalytic conversion during cold engine start-up. More specifically, the invention is directed to an engine exhaust system, generally for automobiles and trucks, that includes molecular sieves positioned within the system to trap hydrocarbons evaporating from the fuel tank and to hold other hydrocarbon pollutants generated by the engine to prevent their discharge into the atmosphere until the catalytic converter in the system reaches an efficient operating temperature for conversion of the hydrocarbons.
As part of the pollution control systems now used, automobiles are equipped with a canister of activated carbon for the purpose of adsorbing hydrocarbon vapors that are emitted from the fuel tank or carburetor by natural evaporation. Hydrocarbon vapors so generated while the engine is idle are vented to the canister to be adsorbed on the activated charcoal and thereby prevented from being emitted to the atmosphere. The charcoal is later purged during engine operation by utilizing the engine vacuum to draw air through the charcoal, thus desorbing the hydrocarbons, after which the air/hydrocarbon mixture is drawn into the engine and burned as a component of the engine's total fuel feed.
Although this system for the adsorption of evaporating hydrocarbons is generally effective, there are inefficiencies based primarily on the geometric arrangement of the activated charcoal in the canister as well as on the amount of charcoal itself. As used today, the activated charcoal is arranged in the canister as a bed of granulated material, and because of the non-uniformity of gas diffusion through the bed, the charcoal often reaches its practical (if not theoretical) saturation point. Any further hydrocarbon vapors generated by evaporation then pass through the bed unadsorbed and are vented to the atmosphere. Moreover, the purge is often not totally effective. Desorption is more efficiently accomplished at an elevated temperature, but since the charcoal system generally operates at ambient temperature, the charcoal may not be fully purged, causing saturation to be reached earlier during the next adsorption period. Although one possible solution is the use of a greater volume of charcoal, cost and space constraints make this undesirable.
Another problem with the emission-control systems in present use in automobiles resides in the fact that the precious metal catalysts used in standard catalytic converter systems are generally ineffective at ambient temperature and must be heated, generally to within the range of 300.degree.-400.degree. C., before they are activated. Typically, the catalyst is heated by contacting it with the high temperature exhaust gases from the engine. Continuous contact with those gases and the exothermic nature of the oxidation reactions occurring at the catalyst combine to elevate and maintain the catalyst temperature. The temperature at which a catalytic converter can convert 50% of carbon monoxide, hydrocarbons, or nitrogen oxides (NOx) passing through it is referred to as the "light-off" temperature of the converter.
In most automotive engines, the amount of carbon monoxide and hydrocarbons in the exhaust gas is higher during start-up than after sustained engine operation because, at the outset, the engine efficiency is low. For example, as noted in U.S. Pat. No. 3,896,616, the amount of carbon monoxide at start-up can be 3-10 percent by volume, or more, (versus 0.5-3 percent during normal engine operation), and the amount of hydrocarbons can typically be about 750-2,000 parts per million (ppm) (versus about 100-750 ppm during normal operation). Accordingly, a significant portion of the total emission generated by the typical automotive engine is generated in the first few minutes of operation. Unfortunately, at start-up, when the catalytic converter is most needed, it can be relatively ineffective because it will not yet have reached a temperature at which it is efficiently active.
There have been numerous suggestions for avoiding the pollution problems inherent in engine start-up. For example, it has been suggested that the catalytic converter be placed as close to the engine as possible so that the exhaust contacts the catalyst before it loses heat to the environment, thereby more quickly raising the temperature of the catalyst to operating level. However, because of limitations of space in most vehicles, locating the entirety of catalyst adjacent the engine is difficult.
U.S Pat. No. 3,896,616 suggests the use of two converters, an initial catalyst, preferably in a converter vessel placed near the engine, and a second, down-stream catalytic converter. It is taught that the initial catalyst, being close to the engine, will reach its effective operating temperature significantly sooner than the main, down-stream catalyst. On cold engine start-up, however, during the period before the initial catalyst reaches its effective temperature, substantial quantities of pollutants would still be discharged to the atmosphere. In addition, because the initial catalyst is positioned close to the engine, it is susceptible to being over-heated resulting in degradation of the metal catalyst.
Accordingly, there remains a need for an engine exhaust system that can effectively reduce evaporative emissions and reduce the amounts of pollutants discharged to the atmosphere during the critical engine start-up period.